The instant invention relates to the transportation of cargos up and down stairs and more specifically to a hand truck and stair ramp system for transporting heavy cargos up and down flights of stairs.
It has been found that most worker injuries resulting from the use of hand trucks on stairs are lower back injuries. These injuries are both acute (sudden trauma) and cumulative (repetitive trauma), although acute injuries are sometimes believed to be the final result of cumulative injuries. It has also been found that lower back injuries are responsible for approximately 35% of the injuries that occur in the work place, and that they are responsible for approximately 80% of the monetary costs and losses associated with work place injuries.
The ergonomic and physiological factors associated with lower back injuries include the following: elevated back compression forces, asymmetrical lifting, partially flexed trunk posture, high inertia forces, overloading, and fatigue. All of these factors are present and magnified in the typical methods of moving loaded hand trucks up and down flights of stairs.
To move a typical 180 pound load up a single step with a standard 25-30 pound hand truck, an operator must bend at the waist to lower the hand truck runners or "stair glides" into contact with the apex of the step and then pull the hand truck upwardly. Unfortunately, friction with the step apex often magnifies the necessary pulling forces, such that as much as 285 pounds of lifting force may be required to move a 180 pound load up a single step. Further, when a worker is pulling in the bent-over position, intervertebral compression can be elevated from a normal 120-150 pounds to as much as 600 pounds. Intervertebral compression above 500 pounds is believed to magnify the likelihood of injury by a multiple of 6. In any case, in order to move a hand truck up a step an operator must step up and back with one foot while preparing to pull the load up to the next succeeding step. As a result, in order to move a load up a flight of steps the load must be pulled repetitively in a sudden "jerking" motion to accomplish the task.